High power amplifiers are prevalent in a number of applications including audio power amplifiers for driving loudspeakers, RF transmitters for communications systems, and driver amplifiers for transmission lines. A typical high power amplification system has a series of cascaded amplification stages, each stage of which successively amplifies a system input until a desired output level is reached at the final output stage. For such power amplification systems, power consumption can be appreciable, even in the first few stages of amplification.
For example, an audio amplifier designed to source a large current into a capacitive load may be biased at voltages greater than 20V. An amplifier used in consumer and professional audio applications may contain a number of stages including audio processing, filtering, gain control, and other stages. If the earlier stages the audio amplifier are biased at the same high voltage as last amplifier, power efficiency is lost if the signal headroom significantly exceeds the amplitude of the expected peak signal. For example, if a class. A amplifier is biased to supply voltage of 20V, but the peak-to-peak signal swing is 1V, much of the amplifier's power will be dissipated in the amplifier's devices rather than in the load.
In particular, multi-stage feedback power amplifiers operate inefficiently because the signal levels in the first stage of the power amplifier typically has much more headroom than the output stage. Therefore, power is wasted in the input stage. Using separately biased amplifiers for each stage of a multi-stage feedback amplifier poses problems. For example, if a multi-stage amplifier requires global feedback to set gain and improve linearity, extra parasitics induced by the coupling of separate stages can cause instability and introduce manufacturing difficulties due to component and temperature variation.
In the field of power amplification systems, what is needed are power-efficient circuits and systems.